1. Field of the Invention
The present invention relates to blade systems for stirring or agitating a medium, and more particularly, is directed towards maximizing the strength of the system with respect to medium-induced stress.
2. Discussion of Background
The movement of a fluid through a container (such as a pipe) is characterized by its viscosity, which can be thought of as a sort of "internal friction" or resistance of the fluid to a change in form. The higher the viscosity, the slower the movement of the fluid. Viscosity tends to decrease as the temperature of the fluid increases, so fluid tends to flow faster at higher temperatures.
Any fluid can be classified as one of two general types: a Newtonian fluid is one whose resistance to the passage of a moving object is wholly due to viscous effects, that is, strictly proportional to the speed of the object. Water and most oils are Newtonian fluids. A non-Newtonian fluid is one whose resistance to the passage of a moving object is not strictly proportional to its speed. Typically, such a fluid has "gel-like" properties, behaving as a solid at low levels of force and a liquid at higher ones. Common examples are jelly and wet cement.
Setting behavior in such diverse non-Newtonian fluids as jelly and wet cement is best characterized by measuring a property called "gel strength": that part of a non-Newtonian fluid's resistance to the passage of a moving object which is attributable to its gel-like nature. In practical terms it is the pressure exerted against an object moving steadily through the fluid, but so slowly that viscous effects are negligible. Gel strength, customarily measured in pounds per square foot (p.s.f). or kilograms per square meter (kg/m.sup.2), is the force required to move a blade or other object through the setting mix at some specified uniform speed, over and above the force which would be required to move it through a non-setting, or Newtonian mix of equal viscosity. Usually a rotating assembly of two or more blades is used; the gel strength is then given by the ratio of shaft torque, corrected for viscosity, to the rotational moment of the blade assembly.
Typically, for any specific gel and stage of development, the gel strength is a sufficiently weak function of speed that, if the speed is held within a specified range, the effect of small variations may usually be neglected. Under uniform conditions of temperature and pressure, the gel strength typically increases with time, following an "S"-shaped curve. A period of little change just after mixing is followed by a roughly exponential increase to some peak or plateau value at which the gel strength levels off again. The timing of this process is highly dependent on batch composition, with even trace impurities sometimes showing a strong influence. Process optimization may thus require close monitoring of the time needed for each new batch to reach some specified gel strength or strengths.
A complication in gel-strength measurement is that mechanical disturbance tends to upset the gelling process; this is why wet cement can be carried for hours in mixing trucks without setting. Blade motion, therefore, must be as slow as possible for accurate gel-strength measurement. Low blade speeds also minimize the effects of viscosity, so that in general the measured gel strength can be used without correction. For most applications, a blade speed of about 2.times.10.sup.-5 to 8.times.10.sup.-5 meters per second is optimum.
Another complication is the tendency of a rotating blade assembly to "cut out a plug" from a setting mixture at some intermediate value of gel strength. A shear zone develops around the blade assembly, so that a cylindrical "plug" of mix, of the same outer radius as the blades, breaks away from the outer mass of mix. While setting continues in the plug and in the outer mass, the slippage disrupts gelling in the shear zone; blades and plug continue to turn, but the readings are falsely low since they reflect only the properties of one small, highly disturbed portion of the sample.
Furthermore, blades have typically been designed for use in media much weaker than the blade material, (such as metal blades for whipping cream, kneading dough, grinding coffee beans, etc.), so that blade failure through excessive medium resistance was not heretofore considered a realistic possibility. This possibility had to be addressed in the present invention because of the extremely broad range of strengths which might be encountered, even in a single sample, during the setting process of a medium such as cement.